Project Summary/Abstract Liposomal drug encapsulation has recently gained significant traction as a therapeutic delivery modality, especially as a means of enhancing localized drug release or direct delivery in cancerous tissues with reduced drug side effects. The majority of ?targeted? liposome technologies feature vesicles displaying ligands or binding proteins that bind to specific cell-surface moieties on target cells to enable selective liposome attachment. Absent any further mechanistic regulation of these systems, liposomes are fated to release contents to the extracellular space near the target cell or enter target cells via endosomal pathways, which limits the type of drug that can be delivered as well as the efficiency of cytoplasmic delivery. A few liposome technologies exist which feature proteins or liposomal chemistries that enable membrane fusion to allow either direct entry of drug payloads into the cytosol or endosomal escape. However, these approaches are greatly limited by their inability to simultaneously target predefined receptors of interest with minimal risk of non- specific delivery to other cell types in heterogeneous tissues or organs, while enabling direct cytosolic entry through membrane fusion between the liposome and the target cell. In this work, we propose to develop a modular, ultra-selective liposomal delivery platform technology which utilizes a system of retargeted viral fusion proteins that bind a target receptor of interest to directly trigger membrane fusion at the cell surface and enable cytosolic payload delivery, while minimizing off-target delivery. As proof-of-concept, we will employ this system to deliver payloads to HER2-positive cells in an expression-dependent manner, to demonstrate ultra-selective delivery to breast cancer cells in heterogeneous cultures.